This invention relates generally to irrigation control systems, and more particularly concerns an improved sensor that is matric potential responsive.
Plant root systems extract moisture from the soil continuously during their lifetime. As water is depleted in the soil adjacent to the root system, voids appear in between the soil particles. Since adjacent undepleted moist soil forms a tight seal, the voids thus formed in the water depleted area create a vacuum or suction, the degree of which depends upon the amount of water removed by the root systems. As the partial vacuum increases, it becomes increasingly difficult for the plant roots to extract the required moisture and transport it to the plant structure. This increased vacuum or suction is known as moisture tension and, in recent soil science terminology, is known as matric potential.
It has been determined that most plant growth and crop yield decreases as a function of the stress resulting from matric potential, the maximum yield occuring at matric potential values between -10 to -50 centibars suction. However, because of the wide differences in the grain size and mix of various soils, e.g. fine sand versus clay, the amount of soil moisture which can be removed by the root system, for a given low matric potential, is quite large; i.e., in fine sand, roots can extract all but a few percent of the contained moisture without exceeding -50 centibar suction. At the other end of the soil scale, the millimicron particles of clay have such a high capillary attraction for water the matric potential may reach -50 centibar suction even though the moisture in the clay is as high as 40%.
The wide difference in soil moisture content, as a function of plant root stress, is of great importance to proper design of sensors for controlling irrigation systems. Unless a soil sensor is responsive to changes in matric potential, each soil type would require a separate sensor calibration; e.g. 7% soil moisture in fine sand having -50 centibar suction, 40% soil moisture in clay having -50 centibar suction. For this situation, sensors measuring resistance change as a function of soil moisture would require a 6 to 1 difference in their calibration in order to control irrigation at -50 centibar suction. The same holds true for heat-diffusion sensors (as described in U.S. Pat. No. 2,718,141, L. C. Richards 1955), U.S. Pat. Nos. 2,343,520 & 2,362,344, Baver (1944), since the heat transfer is a direct function of the soil moisture content.